Generation of ultrahigh field by micro-bubble implosion*

Breaking the 100-MeV barrier for proton acceleration will help elucidate fundamental physics and advance practical applications from inertial confinement fusion to tumor therapy. Herein we propose a novel concept of bubble implosions [1]. A bubble implosion combines micro-bubbles and ultraintense laser pulses of 10²⁰ – 10²² W/cm² to generate ultrahigh fields and relativistic protons. The bubble wall protons undergo volumetric acceleration toward the center due to the spherically symmetric Coulomb force and the innermost protons accumulate at the center with a density comparable to the interior of a white dwarf. Then an unprecedentedly high electric field is formed, which produces an energetic proton flash. Three-dimensional particle simulations confirm the robustness of Coulomb-imploded bubbles, which behave as nano-pulsars with repeated implosions and explosions to emit protons. 
Current laser technology is suitable to experimentally identify bubble implosion by observing proton emissions at relativistic energies, which will be a major breakthrough to crack the 100-MeV barrier. For such experiments, a uniform and well-activated Coulomb field must be created inside the bubbles by laser irradiation of micron-sized bubbles embedded inside a solid target. We have demonstrated in terms of the 2D simulation that a symmetric bubble implosion can be achievable even under a realistic condition of laser-matter interaction.

[1] M. Murakami, A. Arefiev, M. A. Zosa, Generation of ultrahigh field by micro-bubble implosion, Scientific Reports 8 (1) (2018) 7537. doi:10.1038/s41598-018-25594-3.
URL https://doi.org/10.1038/s41598-018-25594-3